Advertisement

Advertisement

Cassini gears up for final fiery plunge into Saturn’s atmosphere

Cassini’s final resting place

NASA/JPL/Space Science Institute

By Andy Coghlan

THE countdown has begun. NASA’s Cassini spacecraft has spent more than a decade orbiting Saturn, dipping in and out of its rings and peering at its moons. But in just one year, it will begin its swansong mission, a daring series of manoeuvres that will bring views of Saturn like never before – and end with the spacecraft plummeting to its death in the gas giant’s atmosphere.

“It’s going to be a tough day,” says project manager Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, California, who presented details of Cassini’s final mission at the European Geophysical Union meeting in Vienna last week.

The so-called Grand Finale will begin next year, following six months studying Saturn’s outer F ring, including 12 close fly-bys of Titan, Saturn’s biggest moon, which sports lakes filled with liquid hydrocarbons.

Advertisement

Cassini’s last act will be its most daredevil mission yet. From April 2017 to September, the probe will zoom 22 times between the planet and the innermost of Saturn’s famous rings. “No spacecraft has flown in that region before, so it’s very exciting,” said Spilker.

The scientific payback will be enormous. Flying closer than ever before – some 64,000 kilometres from the gas giant’s centre – Cassini will gather previously unobtainable data to help solve Saturn’s enduring mysteries. These include such basic questions as whether it has a rocky core and the precise length of its day.

At such close quarters, Cassini can collect unprecedented information about the composition of the planet’s ionosphere and atmosphere. The craft will also collect data on radiation belts girding the planet, and use gravity measurements to give the best estimate yet of the mass of the rings.

“I am especially interested in Cassini’s measurement of the mass of Saturn’s rings,” says Larry Esposito at the Laboratory for Atmospheric and Space Physics in Boulder, Colorado. Knowing the mass will help determine whether the rings were recently created, or whether they are as old as the solar system – Esposito’s favoured idea. “It will be a major scientific discovery either way, with implications for the formation of planets in protoplanetary discs.”

And the images it will send back should be spectacular. “We will get some of the highest resolution images ever obtained of the inner D ring,” says Spilker. “We’ll get wonderful views with the sun high in the sky.”

In terms of derring-do, the only comparable mission was in 2008, when Cassini flew through enormous plumes of warm water 100 kilometres across, gushing from the surface of Saturn’s moon Enceladus.

The risk was worth it, because the data collected from that and subsequent fly-bys showed Enceladus has a salty ocean containing organic material, energy sources and a hydrothermal system at the base of the ocean – all necessary for life.

“As the signal stops, there will be a moment of silence for the loss of a friend we’ve known since 1990“

Ironically, that potential for life is part of why Cassini must die. “We are ending the mission by burning up in Saturn’s atmosphere to protect two worlds, Enceladus and Titan, that might have oceans suitable for life,” Spilker says. “NASA required that we dispose of Cassini… in a way that would protect Enceladus and Titan from a later impact.”

The mission will end on 15 September 2017. “There’s something romantic about the idea of Cassini sailing on forever, but it’s trapped in Saturn orbit and can’t roam the cosmic void,” says Jonathan Lunine of Cornell University in Ithaca, New York. “There’s some solace in thinking that Cassini’s atoms will merge in Saturn’s interior with atoms that have been there since the beginning.”

“On the day, we’ll gather at the Jet Propulsion Laboratory with the Cassini family, and when the signal stops, there will be a moment of silence for the loss of a dear friend we’ve known since 1990,” says Spilker. “We’ll say goodbye with a mixture of sadness and pride.”

This article appeared in print under the headline “Cassini gears up for final fiery plunge into Saturn’s atmosphere”